Lets consider this then as a torque problem. Lets look at what you look like from the front:|dw:1343295702478:dw|
Ok, so you see when you lift your leg, the other leg becomes like a fulcrum while anything pointing out perpendicular from it becomes a lever arm. The weight of your entire leg is pulled down by the force of gravity, and causes a torque to cause you to tip over. So to have a net torque of zero, you move your arm on the other side of your body to counteract it by helping you tip in the other direction so they cancel out. Of course in reality it doesn't take very long to take a step, and a lot of that falling motion can be used to walk forward. So yeah, quick and dirty example, hope that makes sense, I can add more if you're interested.